In the making and curing of tire treads or tire tread and belt assemblies, it has been a common practice to wrap one or more layers of an uncured elastomer such as rubber, with or without reinforcements, such as metal cord reinforced plies or fabric and the like about a cylindrical core. The cylindrical core was selected to have a diameter equal to the desired inside diameter, the cylindrical core, with the elastomeric layers attached is generally placed in the center of a cylindrical core of a larger segmented cylinder, which larger segmented cylinder can include mold faces with desired tread patterns on the interior surface thereof, the diameter of the larger segmented cylinder is reduced until it properly contacts the material wrapped about the inner core apparatus and the entire apparatus is heated. The outer mold face segmented cylinder can, of course, be constantly heated but it is more difficult to heat the inner core about which the rubber is wrapped. As a result, premature curing might then take place. In U.S. Pat. No. 4,207,052 it was disclosed that this type of uneven curing of the elastomer deforms the cured tire tread or track belt assembly, in particular the metal cords within the tread belt can be displaced in such prior art molds due to the large temperature gradient between the inner core and the tread forming outer core. Accordingly, U.S. Pat. No. 4,207,052 showed a plurality of inner segments which fit together to form a generally cylindrical mold face surface to form the inner surface of the tread belt along with a means for retracting said plurality of inner segments toward the axis of said inner cylinder and a means for heating said segments. Additionally, the apparatus included a plurality of outer segments which fit together to form an inner generally cylindrical mold face surface of an outer cylinder, said outer cylinder being coaxial with said inner cylinder and of equal lengths therewith along with the means for retracting the plurality of outer segments away from the common axis of said cylinders and a means for heating said plurality of outer segments. The apparatus further included a pair of rings for sealing said inner cylinder to said outer cylinder at the ends thereof, at least one of said sealing rings being removable so that an uncured tire tread or track belt could be positionable between said plurality of inner segments and said plurality of outer segments wherein said plurality of inner segments was retracted toward said common axis and said plurality of outer segments is retracted away from said common axis.
The advantage of the above-referenced prior art mold for producing a tire tread or track belt assembly was that a uniformity of heat and pressure could be achieved upon the molded assembly.
In the prior art it was common for the radially retractable segments to have curved or arcuate shapes with vertically extending edges or sides. The outer segments could move inwardly and create an annular ring for forming the outer tread surface including the tread lugs. The inner segments would move sequentially radially outwardly in an alternating pattern wherein every other inner tread segment would be extended and then every other retracted inner tread segment could be then extended outwardly thereby forming a ring in the fully extended position thereby providing a mold that would cure the tread belt assembly over a period of time. This time was considered the mold cure cycle. Each radially outer segment and each radially inner segment had edges that were generally vertically extending and parallel to the common axis of both the inner and outer segments. Upon curing the tread the inner segments would then be retracted such that the inner surface of the molded tread could be separated from the first radially inward moving segments while the adjacent inner segments held in the closed position would retrain the tread belt assembly within the tread forming outer segments. Once the first radially inner segments had been moved inwardly, the second set of radially inner segments could be moved inwardly thereby releasing the entire inner surface of the tread. Thereafter the outer tread forming segments could be moved radially outwardly thereby releasing the tread belt assembly from the mold in its entirety. As the tread is removed from the outer segments a device would be used to lift the cured tread belt assembly from the mold.
In this type of prior art tread belt assembly molding the radially inner tread forming surface had equal sized tread segments generally with vertical extending edges that extended generally parallel to the axis of the mold rings when in the closed position. Similarly, the outer segments also had vertically extending or generally parallel side faces such that when the mold closed these faces would abut forming a tight joint between the segments. In principle the above-referenced prior art mold as described in U.S. Pat. No. 4,207,052 works generally well for fairly large tread belt assemblies. However, the application of the forces when one closes the inner segments and outer segments of such a mold is such that all movement works in a radial direction increasing the amount of pressure on the radially inward segments substantially.
It has been an objective of the present invention to provide a tread belt mold that would provide improved stability of both the inner and outer segments as the tread belt assemblies being cured such that a uniformity of curing pressure can be insured around the entire 360° circumference.
A second objective of the present invention is to provide a way in which the radially outer mold segments can be provided with improved structural integrity wherein the parting line between circumferentially adjacent segments extends across the tread lug forming depressions within the outer segments thereby avoiding thin ribs within the groove forming section of the mold to as large an extent as possible.